SynSkin.TM., sold by The Dexter Corporation, Adhesive & Structural Materials Division, Pittsburg, Calif. 94565 U.S.A., is a composite surfacing film that provides aerodynamic smoothness and protects composite surfaces with a resistant, paintable surface. Films of this type produce surfaces with less porosity and less composite fabric mark-through when cocured on composite structures. Additionally, these films minimize core mark-through and core crush in cocured, honeycomb stiffened composite parts. SynSkin.TM. is characterized to provide the following advantages in composite construction:
No time consuming finishing steps, resulting in greater productivity, without damage to sensitive parts. PA1 High quality surfaces free of core mark-through. PA1 Prevents core crush during cure of honeycomb stiffened parts. PA1 Provides durable high quality paintable surfaces. PA1 Compatible with a wide variety of composite prepregs and materials. PA1 Storable at ambient temperatures which provides long out-time to accommodate large composite assembly. PA1 Cocurable, sandable and machinable. PA1 Easy to lay up. PA1 Supplied with lightweight compatible fiber mat carrier.
Prior to SynSkin.TM., conventional composite surfacing films comprised lightweight epoxy film adhesives designed for structural bonding. These adhesives have been adapted secondarily for surfacing applications. They require extensive filling, sanding and leveling to remove surface irregularities. Those extra steps are not only time consuming and costly, but they also frequently damage the composite part.
This conventional (non-SynSkin.TM.) approach involves the application of a low flow adhesive film onto a tool face, bonded to the exterior surface of the part. The purpose of doing this is to translate the smooth and level tool surface characteristics onto the face of the cured composite. These films, however, are ineffective in distributing areas of uneven pressure developed in the part during cure. These areas are caused by prepreg warp and fill yarns and by honeycomb cell contact points. They result in highly irregular surfaces radiated to the surface by imprinting from the composite fabric and honeycomb caused by this uneven pressure developed during cure, and porosity and pitting from resin migration.
Cedarleaf, in an article entitled "NOVEL COMPOSITE SURFACING FILM", Published by SAMPLE (Society For The Advancement Of Material And Process Engineering), volume 35, book 2 of 2 books, entitled "Advanced Materials: The Challenge For The Next Decade," describes SynSkin.TM. and its use in composites. According to the article:
"The development of a resin system for the SynSkin.TM. line of products focused on simultaneously solving the problem of leveling, flow, porosity, and core crush during cure of a composite sandwich. These problems normally occur when occuring carbon fiber prepreg/honeycomb sandwich structure a conventional adhesive surfacing material. PA0 "Leveling is the process of distributing areas of uneven pressure to give a flat surface after cure. Potential causes of uneven pressure are prepreg warp and fill yarns and contact points of the honeycomb cell walls on the prepreg. Honeycomb mark off and its associated problems were the most difficult to overcome." PA0 "Work to incorporate materials into SynSkin.TM. to provide lightning strike protection shows early positive performance and will be pursued. PA0 "Potential applications currently under evaluation range from engine cowlings to helicopter blades, aircraft interior panels, lightning strike exterior parts and composite tools."
The severity of core crush additionally depends upon the cure pressure, the stiffness of the honeycomb core, bevel angle of the core in the sandwich structure, and resin contact and flow properties of the prepreg resin. In addition to the resinous properties of surfacing films, factors which promote core crush are high cure pressure, weak honeycomb, high honeycomb bevel and high resin content with low flow in the prepreg matrix. Although the SynSkin.TM. filled resin films do not totally prevent core crush under severe conditions (i.e., those stated above), their use on honeycomb composite structures significantly minimizes core crush compared to the conventional procedures.
According to Cedarleaf, SynSkin.TM. was being developed to incorporate lightning strike protection. In the article, Cedarleaf stated:
Cedarleaf does not indicate how lightning strike protection was incorporated "into SynSkin.TM.." He indicates that that is an important project for development in SynSkin.TM. structures or compositions.
Traditionally, lightning strike protection is afforded to the conventional prepreg based composite by incorporating thin conductive foils or screens into the surface coating layer of the composite. There are conductive thin metal films and screens that are suitable for use in aircraft prepreg based composites that provide a number of advantages, such as low cost, uniformity, light weight, nonflaking, conformability, superior conductivity, improved repairability, no high resistance crossover points, ability to withstand repeated lightning strikes, and the like features.
One such product is called Astro Strike.TM. Screening, sold by Astroseal Products Manufacturing Corp., Old Saybrook, Conn., 06475. Another is called MicroGrid.RTM. Precision-Expanded Foils, sold by Delker Corp., Branford, Conn. 06405.
However, when they are provided in the outer coating of the composite, a number of problems occur. When using a thin adhesive film as a surface ply between the tool and conductive metal foil, the adhesive flows under cure conditions and does not form a discrete barrier layer on the composite surface. As a result, the conductive foil lays on the exterior of the cured structure. This configuration for the conductive metal foil makes the composite part vulnerable to surface microcracking during thermal cycling, presumably promoted in part by expansion differences between the metal foil and unfilled resin matrix during thermal conditioning. Additionally, because the metal foil lays on top of the composite structure, routine sanding and finishing steps required for painting the exterior creates the probability of damaging the conductive screen.
There is need, as expressed by Cedarleaf, for a composite structure that effectively incorporates lightning strike protection while at the same time affords the surface finishing advantages of SynSkin.TM..